Project Summary/Abstract There are fundamental gaps in our understanding of the neurobiological processes involved in addiction, in particular the complex changes produced by drug-paired cues (conditioned reinforcers) that increase drug-taking behavior and provoke relapse. Identifying novel mechanisms by which conditioned reinforcers modify behavior is essential for future efforts to design new treatments to control drug craving and prevent relapse to addiction. The long-term goal of this work is to further our understanding of the behavioral and neurobiological mechanisms mediating drug-paired stimuli that serve as conditioned reinforcers. The objective of this application is to identify the role of the endogenous enkephalinergic system in regulating the reinforcing properties of cocaine-associated stimuli that are an essential part of cocaine addiction. Enkephalins binds with high affinity to delta-opioid receptors (DOPRs), which are highly expressed within the reward pathway, and DOPR activation can alter responding for drug-paired cues without primary reinforcing effects. Our overarching hypothesis is that enkephalins acting at delta-opioid receptors (DOPRs) in the nucleus accumbens shell (NAc-S) play an essential role in the behavioral and neurochemical mechanisms mediating the conditioned reinforcing properties of cocaine-paired cues. This proposal will couple behavioral measurements of conditioned reinforcement with in vivo microdialysis and comprehensive mass spectrometry analysis to evaluate in real-time the role of the enkephalin and DOPRs in establishing and maintaining the salience of cocaine-associated cues. Through the innovative use of behavioral and neurochemical measurement techniques, this proposal will expand beyond the status quo to investigate novel mechanisms selectively mediating conditioned reinforcement but not necessarily the direct, primary reinforcing effects of cocaine. Ultimately, such knowledge has the potential to develop new treatments for preventing relapse.